Loosely speaking, the \(S_8\)  tension is the difference between more direct measurements of the level of fluctuations in the matter power spectrum on \(\sim \) 8 h\(^{-1}\) Mpc scales and their prediction using \(\Lambda \) CDM and the CMB to normalize. More quantitatively, the directly measured values from KiDS-1000 [1]} and DES Y3 [2]} combined give \(S_8 \equiv \sqrt{\Omega _m/0.3}\, \sigma _8 = 0.769 \pm 0.016\)  which is \(2.9\sigma \)  lower then - for example - the value obtained from the CMB by Planck [3]} \(S_8=0.834\pm 0.016\) . Although this tension is not as statistically significant as the \(H_0\)  tension, it has appeared across many different independent measurements of \(S_8\)  [4]}. Moreover, it raises a basic questions as to what sorts of models might impact the power spectrum at measurable scales, while leaving CDM-like cosmology on larger scales. The fact that a simple model extension immediately points to consequences relevant at the appropriate scale is a striking coincidence.
